A technique for carrying out vector control of an induction motor for driving a railway electric vehicle is described in Japanese application patent laid-open publication No. Hei 5-83976. In a railway electric vehicle, generally in a high speed operating area, in order to decrease the switching loss of an inverter or to utilize the direct current power supply voltage at a maximum efficiency, a control in which a PWM pulse mode becomes a one pulse mode is used.
In the one pulse mode, the size of the voltage can not be controlled, and so a technique has been proposed for carrying out a one pulse mode vector control, as described in a thirty-three times railway cybernetics use domestic symposium paper (1996, November) pp. 247-250 "A vehicle drive system where a vector control is adopted".
In the vector control technique described in Japanese application patent laid-open publication No. Hei 5-83976, the control apparatus includes two current control means for correcting two voltage command signals for the vector control in accordance with a deviation between an exciting current command value and a detected exciting current and in accordance with a deviation between a torque current command value and a detected torque current. In addition to the these two current control means, the apparatus also provides a third current control means for converting a slip frequency. However, since the control system construction becomes complicated, when a command signal is executed using a microcomputer, a problem occurs in that the execution time becomes long.
Further, in the above stated document "A vehicle drive system where a vector control is adopted", when using the one pulse mode, it is necessary to additionally employ a magnetic flux correction value function, namely a feedback for carrying a weak magnetic field. Further, in both conventional techniques, it is necessary to change over the control system between the pulse mode and modes other than the one pulse mode.
In addition to the above-stated two conventional techniques, there is a further technique described in Japanese application patent laid-open publication No. Hei 2-32788. In the vector control system described in this patent laid-open publication, as shown in FIG. 16 thereof, in response to each of the components of an exciting current and a torque current, a voltage command is executed. A command of the torque current provides a current control system for issuing a primary frequency command to suit an actual value and the above-stated voltage command is executed in accordance with the obtained primary frequency command. However, in the vector control described in this publication, since in the high speed operating area the PWM pulse mode becomes a one pulse mode, there is a problem in that, when the voltage control becomes ineffective, the vector control cannot be carried out, and there is no statement which gives consideration to this problem in the publication.
An object of the present invention is to provide a control apparatus for an induction motor and a method of control thereof in which, when vector control is carried out in the induction motor, a more simple control can be obtained; and, further, from a low speed operating area to a high speed operating area where the number of pulses of PWM becomes one pulse, in which the direct current power supply voltage of the PWM inverter is utilized at a maximum, a good vector control can be carried out continuously without a change-over between control means.